In a manufacturing system using a conveyor, it is often necessary to cause some event to occur when a part on the conveyor reaches some point on the conveyor track. One method is to install a part registration sensor (e.g., a photo-eye or proximity switch) where the event is to occur. This, however, is not always feasible due to hostile environments, the desire to trigger multiple events from one registration sensor, or the need for variable trigger points. Whenever an event must be triggered downstream from a part registration sensor, an encoder attached to the line can be used to track the object, and the event triggered based on this encoder tracking.
In a conveyor driven by a chain, an encoder is typically coupled to the chain by a sprocket meshing with the chain. The encoder therefore delivers a fixed number of pulses per revolution, and thus a fixed number of pulses per link, a number that is the same for each link regardless of how the length may vary from one link to the next. In fact, due to manufacturing tolerances and wear, not all links are the same in length. For example, a 4.5 m section of chain of a chain-on-edge conveyor, having perhaps 30 links each approximately 15 cm long, may in fact be kept in service until the same 30 links actually measure 4.57 m in length. Furthermore, a section of damaged or excessively worn chain may be replaced with new chain, while the surrounding chain is still moderately worn, resulting in nominal link lengths that vary significantly from one section of chain to another. Thus, if a downstream event is to be triggered by the tracking of a part based on the number of encoder pulses since the part was registered, the trigger point for an event that is to occur 4.5 m downstream, which would be 30 links not accounting for wear, will in fact occur 7 cm further downstream. Though this discrepancy may be reduced by placing the part registration closer to the downstream event, doing that is often too expensive or just not feasible. What is needed is a process of measuring the wear of the conveyor, and correcting for it, to enable accurate remote triggering.